1. Field
The embodiments described below relate generally to imaging, and may be applied to systems for generating time-based images.
2. Description
Three-dimensional imaging systems are commonly used to generate images of an internal portion of a body. As one example, a computed tomography (CT) system includes an X-ray source and a radiation receiver that are mounted to face one another on opposite sides of a ring. A body is positioned within the ring so that a portion of interest lies between the X-ray source and the radiation receiver. The X-ray source then emits X-ray radiation that passes through the portion of interest and is received by the radiation receiver.
The receiver produces a set of data that represents the attenuative properties of tissues that lie between the X-ray source and the receiver. This set of data comprises a projection image. The ring is then rotated in order to rotate the X-ray source and the radiation receiver around the portion of interest. During the rotation, the X-ray source transmits radiation toward the receiver and the receiver produces projection images corresponding to various rotational angle positions. A three-dimensional image of the portion of interest may be generated from the projection images using known reconstruction techniques.
Most reconstruction techniques assume that the spatial distribution of the internal portion's linear attenuation coefficient is identical for all rotational angle positions. This assumption is not accurate if the portion is in motion during acquisition of the projection images. Therefore, to provide improved three-dimensional imaging of a body in periodic motion (e.g., resulting from breathing motion, heart beat, etc.), some imaging systems acquire multiple sets of projection images, wherein each set corresponds to a different phase of the periodic motion. Conventional systems may acquire five to ten of such sets of projection images.
A set of projection images that corresponds to a first phase of the periodic motion may be used to generate a three-dimensional image of the internal portion as it appears during the first phase. Similarly, a set of projection images that corresponds to a second phase of the periodic motion may be used to generate a three-dimensional image of the internal portion as it appears during the second phase.
The plurality of three-dimensional images may be used to identify a tumor or other target within the patient. According to some techniques, a radiation oncologist views a “slice” of each three-dimensional image. Each slice illustrates a same portion of the patient, but at different phases of the periodic motion. The oncologist indicates a location of the target within each slice using a graphical input device. As a result, the location of the target during each represented phase of the periodic motion is known. A treatment region is then determined based on the geometrical union of the indicated locations. Such a procedure may be unsatisfactorily time and resource-consuming, particularly when the number of represented phases is large.
According to other target-identification techniques, an oncologist indicates a location of a target within one slice of a first three-dimensional image representing a first phase of motion. A mathematical transform is then determined between the first three-dimensional image and a second three-dimensional image representing a second phase of motion. The transform is applied to the indicated location to determine a second location of the target within the second three-dimensional image. Next, a second mathematical transform is determined between the second three-dimensional image and a third three-dimensional image representing a third phase of motion. The second transform is applied to the second location to determine a third location of the target within the third three-dimensional image. The process continues for each subsequent phase of motion. This latter technique may be unsatisfactory due to one or more of: the time taken to determine the transforms; inaccuracy of the transforms; the time taken to apply the transforms; and the inability of the transforms to correctly predict subsequent locations of a target.